The effects of ultraviolet radiation on antibiotic-resistant bacteria in vitro.

Wound infections produced by antibiotic-resistant bacterial strains are particularly difficult to manage. This study examined the effectiveness of ultraviolet (UV) light treatment in killing antibiotic-resistant strains of Staphylococcus aureus and Enterococcus faecalis in vitro. Between 2 and 5 replications of each organism at 10(8) organisms/ml were prepared and plated on sheep blood agar medium and treated with UV light (254 nm, 15.54 mW/cm2 output). Irradiation times were 0, 2, 5, 8, 15, 30, 45, 60, 90 or 120 seconds. Bacterial cultures were then incubated at 35 degrees C for 24 hours. Kill rates were 99.9 percent for the methicillin-resistant strain of S. aureus (MRSA) at 5, 8, 15, 30, 45, 60 seconds and 100 percent at 90 and 120 seconds. Kill rates were 99.9 percent at 5, 8, 15, 30 seconds for vancomycin-resistant E. faecalis (VRE) and 100 percent at 45, 60, 90, 120 seconds. Similar results were found with UV light treatment of the antibiotic-susceptible strains of S. aureus and E. faecalis. A significant difference in kill rates at 30 seconds of UV exposure was detected between the antibiotic-resistant strain of S. aureus and the antibiotic-resistant strain of E. faecalis (Student's t test, p < 0.01). Significant differences were also detected in the kill rates at 30 second exposure times for the antibiotic-susceptible strains of S. aureus and E. faecalis. These findings suggest that the Enterococcal bacteria is more susceptible to the killing effects of UV. This data also suggests that UV light at 254 nm is bactericidal for antibiotic-resistant strains of S. aureus and E. faecalis at times as short as 5 seconds and that the enterococcal bacteria is more susceptible to the killing effects of UV. With recommended patient treatment times for infected wounds being significantly longer than 5 seconds, this data indicates that patient treatment times need to be re-examined.